Process and apparatus for forming nanoparticles using radiofrequency plasmas

ABSTRACT

Methods and apparatus for producing nanoparticles, including single-crystal semiconductor nanoparticles, are provided. The methods include the step of generating a constricted radiofrequency plasma in the presence of a precursor gas containing precursor molecules to form nanoparticles. Single-crystal semiconductor nanoparticles, including photoluminescent silicon nanoparticles, having diameters of no more than 10 nm may be fabricated in accordance with the methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/581,141, filed Jun. 18, 2004, and from U.S.Provisional Patent Application Ser. No. 60/623,979, filed Nov. 1, 2004,the entire disclosures of which are incorporated herein by reference.

STATEMENT OF GOVERNMENT INTERESTS

This invention was made with United States government support awarded bythe National Science Foundation under Grant No. DGE 0114372 and DMR0212302. The United States government has certain rights in thisinvention.

FIELD OF THE INVENTION

This invention relates to radiofrequency plasma-based processes for usein the production of nanoparticles made from a variety of materials,including semiconductors, and to radiofrequency plasma reactors adaptedto produce the nanoparticles.

BACKGROUND

Nanoparticles have recently attracted significant attention fromresearchers in a variety of disciplines, due to a wide array ofpotential applications in the fabrication of nanostructured materialsand devices. Semiconductor nanoparticles, such as silicon nanoparticles,are of special interest due to their potential uses inphotoluminescence-based devices, doped electroluminescent lightemitters, memory devices and other microelectronic devices, such asdiodes and transistors. Different methods have been used to synthesizefree standing silicon nanoparticles. These methods include laserpyrolysis of silane, laser ablation of silicon targets, evaporation ofsilicon and gas discharge dissociation of silane.

Amorphous and polycrystalline silicon particles can be produced usingargon-silane discharges. See for example, U.S. Pat. No. 4,583,492.However such particles are not suitable for many device applicationswhich require single-crystal particles. Single-crystal nanoparticleshave higher carrier velocities due to the absence of grain boundaries ordefects leading to potentially better performance. Hence a reproduciblereliable method for generating monodisperse, single-crystalnanoparticles is highly desirable for device applications.

Recently single-crystal silicon nanoparticles have been made usingvery-high-frequency (VHF) pulsed gas plasmas. Using these VHF pulsed gasplasmas it has been reported that single nanocrystals of silicon havingdiameters of about eight nanometers (nm) may be formed. See, forexample, Ifuku et al., Jpn. J. Appl. Phys., 36 (1997), 4031-4034.

SUMMARY

Plasma-based methods for producing nanoparticles are provided. Themethods include the step of generating a radiofrequency (RF) plasma inthe presence of a precursor gas containing precursor molecules. Themethods are well suited for the production of single-crystalsemiconductor nanoparticles, such as silicon nanoparticles, fromprecursor molecules, such as silane.

Using the methods provided herein, nanoparticles having a variety ofdiameters may be fabricated. In some instances the plasma conditions maybe tailored to provide single-crystal semiconductor nanoparticles havingan average diameter of no more than about 10 nm. Such nanoparticles havebeen shown to exhibit photoluminescence.

In some embodiments, the nanoparticles include an organic or inorganicpassivation layer. The inorganic passivation layer may compriseinorganic materials other than natural oxides of the semiconductor. Forexample, silicon nanoparticles having a silicon nitride or siliconcarbide layer on their outer surface may be produced in accordance withthe present methods.

The RF plasma may be generated in a plasma reactor having an electrodeassembly that includes two electrodes arranged in a substantiallyparallel alignment and separated by a gap where at least one of the twoelectrodes is a ring electrode. Although other electrode geometries andconfigurations may also be used. Production of nanoparticles may beincreased by using a plasma apparatus having multiple plasma chambersoperating in parallel. For example, in one such apparatus multipleplasma reactors (e.g., quartz plasma tubes) may be arranged in asubstantially parallel arrangement inside a larger vacuum vessel and aseparate plasma discharge may be ignited and sustained in each reactor.Alternatively, multiple substantially parallel plasma reactors may bedefined by holes through a dielectric medium sandwiched between twoelectrodes.

Crystalline semiconductor nanoparticles produced in accordance with thepresent processes and apparatus are suited for use in a wide variety ofdevice applications. Such applications include, but are not limited to,memory devices (e.g., flash memory devices), light emitting devices,including both photo- and electroluminescent devices (e.g., siliconlight emitting diodes and lasers), display devices (e.g., organic LEDs),sensors and other microelectronic devices, such as diodes andtransistors, including Schottky barrier field effect transistors (FETs).Examples of devices into which the nanoparticles of the presentinvention may be incorporated are described in U.S. Patent ApplicationPublication No. 2003/0034486 and in U.S. patent application Ser. No.60/458,942 entitled, “Light Emitting Ceiling Tile,” the entiredisclosures of which are hereby incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an apparatus for producingnanoparticles.

FIG. 2 shows a schematic diagram of the electrode assembly used in theapparatus FIG. 1.

FIG. 3 shows a transmission electron microscope (TEM) image ofsingle-crystal silicon nanoparticles produced according to Example 1.

FIG. 4 shows the diffraction pattern for the nanoparticles of FIG. 3.

FIG. 5 shows a graph of the size distribution of the single-crystalsilicon nanoparticles produced according to Example 1.

FIG. 6 shows a graph of the size distribution of the siliconnanoparticles produced according to Example 2.

FIG. 7 shows a TEM image of a single-crystal silicon nanoparticleproduced according to Example 2.

FIG. 8 shows a TEM image of the single-crystal silicon nanoparticlesproduced according to Example 3.

FIG. 9 shows a TEM image of an individual single-crystal siliconnanoparticle produced according to Example 3.

FIG. 10 shows a graph of the size distribution of the siliconnanoparticles produced according to Example 3.

FIG. 11 is a schematic diagram of a three-phase plasma apparatus thatincludes three reaction chambers in series. The first chamber is aplasma reactor, the second chamber may be either a plasma or gas-phasereactor and the third reactor is a liquid phase reactor.

FIG. 12 is a schematic diagram showing the apparatus of FIG. 11 wherethe third reactor has been replaced by a moving substrate.

FIG. 13 is a schematic diagram of a parallel plasma reactor.

FIG. 14 is a schematic diagram of a parallel plasma reactor made from acomposite sheet composed of a dielectric material sandwiched between anRF electrode and a ground electrode.

FIG. 15 shows a dark-field transmission electron micrograph ofphotoluminescent silicon nanoparticles made in accordance with Example4.

FIG. 16 shows a dark-field transmission electron micrograph ofphotoluminescent silicon nanoparticles made in accordance with Example4.

FIG. 17 shows the photoluminescence spectra for crystalline Sinanoparticles synthesized in accordance with Example 4, below.

FIG. 18 shows a dark-field transmission electron micrograph ofphotoluminescent silicon nanoparticles made in accordance with Example6, below.

FIG. 19 shows the photoluminescence spectra for crystalline Sinanoparticles synthesized in accordance with Example 6, below.

FIG. 20 is a schematic diagram of a two-phase plasma apparatus thatincludes two reaction chambers in series.

DETAILED DESCRIPTION

RF plasma-based processes for the fabrication of nanoparticles areprovided. Some embodiments of the processes use a constricted RF plasmato produce the nanoparticles. More specifically, these processes utilizean RF plasma operated in a constricted mode to produce nanoparticlesfrom a precursor gas. Although the constricted plasmas may be used toproduce nanoparticles composed of a broad range of materials, theprocess is well-suited for the production of small single-crystalsemiconductor nanoparticles. Other embodiments of the processes arecapable of producing single crystalline nanoparticles using a RF plasmathat is not operated in a constricted mode. As such, the plasma-basedprocesses provided herein are substantially different from moreconventional RF plasma-based approaches which utilize stable, uniformplasmas to produce amorphous or polycrystalline semiconductor particles.

The process may be carried out by introducing a precursor gas and,optionally, a buffer gas into a plasma chamber and generating an RFcapacitive plasma in the chamber. In certain embodiments the RF plasmais created under pressure and RF power conditions that promote theformation of a plasma instability (i.e., a spatially and temporallystrongly non-uniform plasma) which causes a constricted plasma to formin the chamber. The constricted plasma, sometimes also referred to ascontracted plasma, leads to the formation of a high-plasma densityfilament, sometimes also referred to as a plasma channel. The plasmachannel is characterized by a strongly enhanced plasma density,ionization rate, and gas temperature as compared to the surroundingplasma. It can be either stationary or non-stationary. Sometimesperiodic rotations of the filament in the discharge tube may beobserved, sometimes the filament randomly changes its direction ofrotation, trajectory and frequency of rotation. The filament may appearlongitudinally non-uniform, sometimes termed striated. In other cases itmay be longitudinally uniform. Mechanisms leading to the formation ofthe filament include, but are not limited to, thermal instabilities,non-linear ionization behavior, and the presence of electron attachingspecies such as negative ions or particles.

An inert buffer or carrier gas, such as neon, argon, krypton or xenon,may desirably be included with the precursor gas. The inclusion of suchgases in the constricted plasma-based methods is particularly desirablesince these gases promote the formation of the thermal instability toachieve the thermal constriction. In the RF plasmas, dissociatedprecursor gas species (i.e., the dissociation products resulting fromthe dissociation of the precursor molecules) nucleate and grow intonanoparticles.

For illustrative purposes only, much of the discussion that followsdescribes the methods in the context of crystalline semiconductornanoparticle formation. However, it should be understood that thisdiscussion, including the processes, processing conditions andapparatus, also apply to the production of other types of nanoparticles,including, but not limited to, metal nanoparticles, metal alloynanoparticles, metal oxide nanoparticles, metal nitride nanoparticlesand ceramic nanoparticles.

Without wishing or intending to be bound to any particular theory of theinvention, the inventors believe the formation of a constricted RFplasma promotes crystalline nanoparticle formation because theconstricted plasma results in the formation of a high current densitycurrent channel (i.e., filament) in which the local degree ofionization, plasma density and gas temperature are much higher thanthose of ordinary diffuse plasmas which tend to produce amorphousnanoparticles. For example, in some instances gas temperatures of atleast about 1000 K with plasma densities of up to about 10¹³ cm⁻³ may beachieved in the constricted plasma. Additional effects could lead tofurther heating of the nanoparticles to temperatures even higher thanthe gas temperature. These include recombination of plasma electrons andions at the nanoparticle surface, hydrogen recombination at the particlesurface and the condensation heat release related to nanoparticlesurface growth. In some instances the nanoparticles may heated totemperatures several hundred degrees Kelvin above the gas temperature.Together, it is believed, these effects lead to the rapid formation ofcrystalline nanoparticles. These conditions are so well-suited for theproduction of small crystalline nanoparticles, that the plasma may becontinuous, rather than a pulsed plasma.

Thus, some embodiments of the present processes use an RF plasmaconstriction to provide high gas temperatures using relatively lowplasma frequencies. This is advantageous because it allows for theproduction of crystalline semiconductor nanoparticles using RF plasmaequipment which is quite readily available and reasonably priced. Incontrast, other plasma processing methods increase the frequency of thedischarge (e.g., by using a VHF discharge or a microwave discharge) inorder to provide higher plasma densities, which in turn provide higherparticle temperatures. Such processes may require the use of lessreadily available high frequency plasma equipment. In addition, higherfrequency plasma processes are typically carried out in a pulsed plasmamode in order to control particle growth rates. In contrast theprocesses of the present invention may be carried out in a continuousplasma mode, making the present processes more suitable for scaling upto high throughput production.

Conditions that promote the formation of a constricted plasma may beachieved by using sufficiently high RF powers and gas pressures whengenerating the RF plasma. Any RF power and gas pressures that result inthe formation of a constricted RF plasma capable of promotingnanoparticle (e.g., crystalline semiconductor nanoparticles) formationfrom dissociated precursor gas species may be employed. Appropriate RFpower and gas pressure levels may vary somewhat depending upon theplasma reactor geometry. However, in one illustrative embodiment of theprocesses provided herein, the RF power used to ignite the RF plasmawill be at least about 100 Watts and the total pressure in the plasmachamber in the presence of the plasma (i.e., the total plasma pressure)will be at least about 1 Torr. This includes embodiments where the RFpower is at least about 110 Watts and further includes embodiments wherethe RF power is at least about 120 Watts. This also includes embodimentswhere the total pressure in the plasma chamber in the presence of theplasma is at least about 5 Torr and further includes embodiments wherethe total pressure in the plasma chamber in the presence of the plasmais at least about 10 Torr (e.g., about 10 to 15 Torr).

Conditions that promote the formation of a non-constricted RF plasmasmay be similar to those described above for the production ofconstricted plasmas, however, nanoparticles are generally formed in thenon-constricted plasmas at lower pressures, higher precursor gas flowrates, and lower buffer gas flow rates. For example, in some embodimentsnanoparticles (e.g., silicon nanoparticles) are produced in an RF plasmaat a total pressure less than about 5 Torr and, desirably, less thanabout 3 Torr. This includes embodiments where the total pressure in theplasma reactor in the presence of the plasma is about 1 to 3 (e.g., 1.5to 2) Torr. Typical flow rates for the precursor gas in theseembodiments may be at least 5 sccm, including embodiments where the flowrate for the precursor gas is at least about 10 sccm. Typical flow ratesfor buffer gases in these embodiments may be about 1 to 50 sccm (e.g.,about 15 to 25 sccm).

The frequency of the RF voltage used to ignite the radiofrequencyplasmas may vary within the RF range. Typically, however, a frequency of13.56 MHz will be employed because this is the major frequency used inthe RF plasma processing industry. However, the frequency will desirablybe lower than the microwave frequency range (e.g., lower than about 1GHz). This includes embodiments where the frequency will desirably belower than the very high frequency (VHF) range (e.g., lower than about30 MHz). For example, the present methods may generate radiofrequencyplasmas using radiofrequencies of 25 MHz or less.

Using the processes provided herein nanoparticles having very smallsizes may be produced. As used herein, the term “nanoparticle” generallyrefers to particles that have an average diameter of no more than about100 nm. Nanoparticles have an intermediate size between individual atomsand macroscopic bulk solids. Nanoparticles typically have a size on theorder of the Bohr exciton radius (e.g., 4.9 nm for silicon), or the deBroglie wavelength, of the material, which allows individualnanoparticles to trap individual or discrete numbers of charge carriers,either electrons or holes, or excitons, within the particle. The spatialconfinement of electrons (or holes) by nanoparticles is believed toalter the physical, optical, electronic, catalytic, optoelectronic andmagnetic properties of the material. The alterations of the physicalproperties of a nanoparticle due to confinement of electrons aregenerally referred to as quantum confinement effects.

Nanoparticles may exhibit a number of unique electronic, magnetic,catalytic, physical, optoelectronic and optical properties due toquantum confinement effects. For example, many semiconductornanoparticles exhibit photoluminescence effects that are significantlygreater than the photoluminescence effects of macroscopic materialshaving the same composition. One method for reducing the size of thenanoparticles is to reduce the residence time of the nanoparticles inthe RF plasma which reduces the growth time for the nanoparticles.

In some instances, the nanoparticles produced in accordance with theprocesses provided herein may have an average diameter of no more thanabout 50 nm. This includes nanoparticles having an average diameter ofno more than about 30 nm, further includes nanoparticles having anaverage diameter of no more than about 10 nm, still further includesnanoparticles having an average diameter of no more than about 5 nm andeven further includes nanoparticles having an average diameter of nomore than about 2 nm.

In some instances the methods may be used to produce photoluminescentsingle-crystal semiconductor nanoparticles. For example,photoluminescent single-crystal silicon nanoparticles having a diameterof less than about 5 nm may be produced in accordance with the processesprovided herein. The color of the light emitted by thesephotoluminescent nanoparticles will depend on the size of thenanoparticles, with smaller nanoparticles emitting at higher energiesthan larger nanoparticles. In some instances the resulting nanoparticlesmay be freestanding, highly oriented, relatively monodisperse and/orsubstantially nonagglomerated.

The size of the nanoparticles produced in accordance with the presentprocesses will depend, at least in part, on the residence time of theparticles in the plasma, which in turn depends on the flow rate of theprecursor gas and the total pressure in the plasma chamber duringparticle formation. However, the total pressure may also effect thedegree of crystallinity of the resulting nanoparticles. Lower totalpressures tend to produce smaller particles because they allow for morediffusion losses of precursor molecules and thus lead to a low growthrate of the particles and, therefore, tend to be favored for RFplasma-based processes that do not use constricted plasmas. For themethods that employ RF plasmas, on the other hand, higher totalpressures tend to promote the plasma instability and constriction thatproduces single-crystal particle formation. Thus, for these methods, thetotal pressures used to create small single-crystal nanoparticles shouldstrike a balance between these considerations.

The inventors have discovered that increased gas flow rates and totalplasma reactor pressures may be used to produce nanoparticles, includingsingle-crystal semiconductor (e.g., silicon) nanoparticles, havingaverage diameters of no more than about 10 nm and desirably no more thanabout 5 nm. For example, in some embodiments of the plasma processes,total gas flow rates of at least 30 standard cubic centimeters (sccm),precursor gas flow rates of at least about 0.01 sccm and total plasmapressures of at least about 1 Torr may be used to produce single-crystalsemiconductor nanoparticles having average particle diameters of no morethan about 10 nm. This includes embodiments where total gas flow ratesof at least about 200 sccm and total plasma pressures of at least about10 Torr are used and further includes embodiments where total gas flowrates of at least about 50 sccm, precursor gas flow rates of at leastabout 1 sccm, and total plasma pressures of at least about 1 Torr areused. In addition, shorter residence times, and thereby smallernanoparticles, may be achieved by reducing the size of the plasmachamber.

The precursor gas contains precursor molecules that may be dissociatedto provide precursor species that form nanoparticles in a RF plasma.Naturally, the nature of the precursor molecules will vary dependingupon the type of nanoparticles to be produced. For example, to producesemiconducting nanoparticles, precursor molecules containingsemiconductor elements are used. A discussion of some of the varioustypes of nanoparticles that may be produced using RF plasmas inaccordance with the present methods, including examples of suitableprecursor molecules, is provided below.

The precursor gas is desirably mixed with a buffer gas that acts as acarrier gas and, in some embodiments, helps to provide a plasmaconstriction. The buffer gas is desirably an inert gas (e.g., a raregas) with a low thermal conductivity and a high molecular weight (insome instances, higher than that of the precursor molecules). Neon,argon, krypton and xenon are examples of suitable buffer gases. Thebuffer gas may be characterized in that a non-uniform constricteddischarge with a stationary current channel may be formed in the buffergas alone. This stationary current channel becomes a rotating currentchannel when the precursor gas is added to the plasma chamber. Thefrequency of the rotation and, therefore, the residence times of thenanoparticles in the current channel, depends on the flow rate of theprecursor gas.

Semiconductor Nanoparticles:

Using the present processes, nanoparticles composed of Group IV, GroupIV-IV, Group II-VI and Group III-V elements may be prepared fromprecursor molecules containing these elements.

The processes provided herein are particularly well-suited for use inthe production of single-crystal nanoparticles composed of Group IVsemiconductors, including silicon, germanium and tin, from precursormolecules containing these elements. Silane and germane are examples ofprecursor molecules that may be used in the production of silicon andgermanium nanoparticles, respectively. Organometallic precursormolecules may also be used. These molecules include a Group IV metal andorganic groups. Organometallic Group IV precursors include, but are notlimited to organosilicon, organogermanium and organotin compounds. Someexamples of Group IV precursors include, but are not limited to,alkylgermaniums, alkylsilanes, alkylstannanes, chlorosilanes,chlorogermaniums, chlorostannanes, aromatic silanes, aromatic germaniumsand aromatic stannanes. Other examples of silicon precursors include,but are not limited to, disilane (Si₂H₆), silicon tetrachloride (SiCl₄),trichlorosilane (HSiCl₃) and dichlorosilane (H₂SiCl₂). Still othersuitable precursor molecules for use in forming crystalline siliconnanoparticles include alkyl and aromatic silanes, such as dimethylsilane(H₃C—SiH₂—CH₃), tetraethyl silane ((CH₃CH₂)₄Si) and diphenylsilane(Ph-SiH₂-Ph). In addition to germane, particular examples of germaniumprecursor molecules that may be used to form crystalline Genanoparticles include, but are not limited to, tetraethyl germane((CH₃CH₂)₄Ge) and diphenylgermane (Ph-GeH₂-Ph).

Group IV-IV nanoparticles may be formed by exposing a precursor gascontaining a mixture of one or more Group IV element-containingprecursor molecules and one or more different Group IVelement-containing precursor molecules, to a plasma to dissociate theprecursor molecules into precursor species and exposing the resultingprecursor species to a RF plasma to form crystalline Group IV-IVnanoparticles. Examples of Group IV-IV nanoparticles that may beproduced in accordance with the present invention include SiC, SiGe andSiSn.

As noted above, crystalline semiconductor nanoparticles that includemixtures of elements from different groups may be formed. One commongroup of semiconductor materials are the Group II-VI semiconductors.These materials are composed of mixtures of Group II metals and Group VImetals. Group II-VI nanoparticles may be formed by exposing a precursorgas containing a mixture of one or more Group II element-containingprecursor molecules and one or more Group VI element-containingprecursor molecules to a plasma to dissociate the precursor moleculesinto precursor species and exposing the resulting precursor species to aRF plasma to form crystalline Group II-VI nanoparticles. Dimethylcadmium is one example of a suitable Group II organometallic precursormolecule and diphenyldiselenide is one example of a suitable Group VIorganometallic precursor.

Group III-V nanoparticles may be formed by exposing a precursor gascontaining a mixture of one or more Group III element-containingprecursor molecules and one or more Group V element-containing precursormolecules, to a plasma to dissociate the precursor molecules intoprecursor species and exposing the resulting precursor species to a RFplasma to form crystalline Group III-V nanoparticles. Trimethyl galliumis one example of a suitable Group III organometallic precursor moleculeand trimethyl arsenide is one example of a suitable Group Vorganometallic precursor. In another example, indium-gallium-arsenidenanoparticles may be formed by exposing a precursor gas containing amixture of gallium-containing precursor molecules (e.g., trimethylgallium). arsenic-containing precursor molecules (e.g., trimethylarsenide) and indium-containing precursor molecules (e.g., trimethylindium) to the RF plasma. Other crystalline nanoparticles that may beformed include, but are not limited to, indium-phosphide,indium-arsenide, and gallium-phosphide nanoparticles.

Nitrides of Group III elements, including gallium nitride, are anotherexample of Group III-V nanoparticles that may be produced. Thesenitrides may be formed using nitriding agents, such as NH₃, N₂H₄ and N₂,as precursor molecules.

Metal Nanoparticles:

Using the present processes, metal nanoparticles, includingnanoparticles composed of Group IIA, IIIA, IVA, VA, IB, IIB, IVB, VB,VIB, VIIB and VIIIB metals, may be prepared from precursor moleculescontaining these elements. When the present processes are used toproduce metal nanoparticles, hydrogen gas may be included as a reducingagent in the precursor gas in order to avoid unwanted oxide formationand to minimize the incorporation of impurities into the nanoparticles.

Suitable metal precursor molecules include metal carbonyls, such asGroup VIB metal carbonyls (e.g., Cr(CO)₆, Mo(CO)₆, W(CO)₆ and the like);Group VIIB metal carbonyls (e.g., Mn₂(CO)₁₀ and Re₂(CO)₁₀); Group VIIIBmetal carbonyls (e.g., Fe(CO)₅, Fe₃(CO)₁₂, Ru(CO)₅, Os(CO)₅, H₂Os(CO)₄,Co₂(CO)₈, [RhCl(CO)₂]₂, and Ni(CO)₄ [gas]). Other metal precursorsinclude metal halide precursors such as Group IVB metal halides (e.g.,TiCl₄ [liquid] and TiI₄); Group VB metal halides (e.g., VCl₄ [liquid]);Group VIB metal halides (e.g., WF₆ [gas] and MoF₆ [liquid]); and GroupVIIIB metal halides (e.g., NiCl₂).

Organometallic precursors may also be employed. The use oforganometallic precursors is advantageous because these precursors allowa wide range of metals to be introduced into the gas-phase. The organicgroups of the organometallic precursors are given the followingabbreviations: acac=acetylacetonate anion (CH₃COCHCOCH₃);hfac=hexafluoroacetylacetonate anion (CF₃COCHCOCF₃);thd=2,2,6,6-tetramethylheptane-3,5-dionate (C₁₁H₁₉O₂);tfac=trifluoroacetylacetonate anion (CF₃COCHCOCH₃); cp=cyclopentadieneanion (C₅H₅); cod=1,5-cyclooctadiene (C₈H₈). Specific examples oforganometallic precursors include Group IIA organometallics (e.g.,Mg(hfac)₂, Ca(acac)₂ and Sr(acac)₂); Group IVB organometallics (e.g.,Ti(tfac)₄ and Hf(tfac)); Group VB organometallics (e.g., V(acac)₃);Group VIB organometallics (e.g., Cr(acac)₃, Cr(cp)₂ and Mo(C₃H₅)₄);Group VIIB organometallics (e.g., Mn(cp)(CO)₃); Group VIIIBorganometallics (e.g., Fe(tfac)₃, Ru(cp)₂, Ru(acac)₃ Co(acac)₃,Rh(C₃H₅)₃, Ir(acac)₃, Ir(C₆H₇)(cod); Pd(acac)₂, Pd(C₃H₅)(hfac),Pt(hfac)₂ and Pt(acac)₂); Group IB organometallics (e.g., Cu(hfac)₂,Ag(cod)(hfac) and Au(CH₃)₂(acac)); Group IIB organometallics (e.g.,Zn(CH₃)₂, Zn(acac)₂, Cd(C₂H₅)₂ and Hg(CH₃)₂); Group IIIA organometallics(e.g., Al(CH₃)₂, Ga(CH₃)₃, In(CH₃)₂ and In(acac)₃); Group IVAorganometallics (e.g., Sn(CH₃)₄ and Pb(C₂H₅)₄); and Group VAorganometallics (e.g., Bi(C₆H₅)₃).

Metal Alloy Nanoparticles:

Using the present processes, metal alloy nanoparticles may be preparedfrom precursor molecules containing a combination of metal elements.Thus, metal alloy nanoparticles may be produced in a RF plasma ignitedin a mixture of two or more of the metal nanoparticles precursors listedabove. For example, a brass alloy (such as cartridge brass), may beproduced by incorporating 70% of an appropriate copper precursor, suchas Cu(hfac)₂, with 30% of an appropriate zinc precursor, such asZn(acac)₂, to produce particles with a composition nominally equivalentto that of cartridge brass (70:30 copper to zinc).

Metal Oxide Nanoparticles:

Using the present processes, metal oxide nanoparticles, includingnanoparticles of simple metal oxides (e.g., iron oxide) and mixed metaloxides (e.g., indium tin oxide or strontium titanate (SrTiO₃)), may beprepared from precursor gases containing a mixture of metal precursormolecules and one or more oxidizing agents. Oxygen is well-suited foruse as an oxidizing agent due to minimization of contamination. However,other oxidizing agents, such as CO₂, NO₂, NO or N₂O may also be used andare particularly suited for use at high reaction temperatures. Otheroxygen bearing gases that are not traditionally considered oxidizingagents may also be used in the plasma provided significant dissociationoccurs. A primary example of such an agent is H₂O, which may reactthrough both oxidation and hydrolysis reactions in the plasma.

The above-listed metal precursors in combination with oxidizing agents,may produce metal oxide nanoparticles in a RF plasma. Other types ofprecursors that may be used in the formation of metal oxide particlesinclude metal alkoxides. Metal alkoxides are compounds with —OR groups,where R is an organic group, such as methyl (CH₃), ethyl (C₂H₅), phenyl(C₆H₅), etc. Due to the large number of organic groups, many variantsexist. Metal alkoxides exist for Group I through Group IV elements. Somenon-limiting examples are Ti(OR)₄, VO(OC₂H₅)₃, Al(OC₄H₉)₃, Hf(OC₂H₅)₄,Ni(OC₂H₅)₄, Ta(OCH₃)₅, Sn(OC₄H₉)₄, and Zr(OC₂H₅)₄.

Metal Nitride Nanoparticles:

Using the present processes, metal nitride nanoparticles, such astitanium nitride nanoparticles, may be prepared from precursor gasescontaining a mixture of metal precursor molecules and one or morenitriding agents. Examples of nitriding agents include NH₃, N₂H₄ and N₂.When nitrogen gas is used as the nitriding agent, it may be used alone,or in combination with hydrogen gas. Halide and organometallicprecursors are examples of precursors that may be used for metal nitridenanoparticle formation. In the halide precursor case, the inclusion ofhydrogen in the precursor gas (either added to the gas stream or fromthe decomposition of a hydrogen-bearing nitriding agent) is desirable.

Ceramic Nanoparticles:

Using the present processes, a wide range of ceramic nanoparticles maybe produced. These include the metal oxides listed above and otherinorganic oxides (e.g., B₂O₃, SiO₂, GeO₂, and SnO₂); the metal nitrideslisted above and other inorganic nitrides (e.g., BN, Si₃N₄, and Ge₃N₄);carbides (e.g., B₄C, Cr₃C₂, Mo₂C, SiC, TaC, TiC, WC, VC, and ZrC);borides (e.g., CrB₂, LaB₆, W₂B, and TiB₂); and silicides (e.g., CrSi₂,MoSi₂, PtSi, WSi₂, ZrSi₂).

These ceramic nanoparticles may be produced in a RF plasma usingmixtures of precursors, oxidizing agents and/or nitriding agents. Forexample, appropriate precursors for the production of boron-containingceramics include BF₃ (gas), BCl₃ (gas), and B₂H₆ (gas). Precursors forsilicon-containing ceramics (including those already listed elsewhere inthis disclosure) include SiH₄, Si₂H₆, SiCl₄, SiH₂Cl₂, SiF₄, Si(CH₃)₄,and silicon alkoxides (such as Si(OCH₃)₄ and Si(OC₂H₅)₄). Precursors forgermanium-containing ceramics include GeH₄, GeCl₄, GeF₄, Ge(CH₃)₄, andgermanium alkoxides (such as Ge(OCH₃)₄). Precursors forcarbon-containing ceramics include many organic molecules in the form ofgases or volatile liquids. The simplest example of such is methane(CH₄).

Oxide-containing ceramic nanoparticles can be formed by combining theappropriate metal or inorganic precursor or precursors with an oxidizingagent, as described above for metal oxides. Similarly,nitride-containing ceramic nanoparticles can be formed by combining theappropriate metal or inorganic precursor or precursors with a nitridingagent, as described above for metal nitrides. Carbon-containing ceramicnanoparticles can be formed by combining the appropriate metal orinorganic precursor or precursors with a carbon precursor, such asmethane, in a RF plasma. In the case of halide precursors, hydrogen mayoptionally be included in the precursor gas to reduce the precursor inthe plasma.

For illustrative purposes, a few reaction mechanisms for the productionof various ceramic nanoparticles is presented. For example, carbidenanoparticles may be produced according to the following mechanisms:WF₆+CH₄+H₂→WC+6HF; andSiH₄+CH₄→SiC+4H₂

Boride nanoparticles can be formed by combining an appropriate metal orinorganic precursor or precursors with a boron precursor, such as BF₃,BCl₃, or B₂H₆, under RF plasma conditions. For example TiB₂nanoparticles may be produced according to the following mechanism:TiCl₄+2BCl₃+5H₂→TiB₂+10HCl

Silicide nanoparticles can be formed by combining an appropriate metalor inorganic precursor or precursors with a silicon precursor, such asSiH₄, under RF plasma conditions. For example, WSi₂ nanoparticles may beproduced according to the following mechanism:WF₆+SiH₄+H₂→WSi₂+HF

Semiconductor nanoparticles produced in accordance with the presentprocesses may optionally be passivated either by using an organic layercovalently attached to the nanoparticle surface or by growing aninorganic passivation layer on the nanoparticle surface to providecore-shell semiconductor nanoparticles. These passivation layers mayplay an important role in preventing reactive degradation of thenanoparticles when exposed to water and oxygen or other chemicalcontaminants. Photoluminescent semiconductor nanoparticles passivatedwith thin layers (e.g., monolayers) of these passivating agents may emitwith relatively short (e.g., nanosecond scale or even sub-picosecondscale) lifetimes and high quantum yields. Examples of organicpassivating agents that may be used to passivate the surfaces include,but are not limited to, alkenes, alkynes, alcohols, alkoxides,carboxylic acids, carboxylates, silanols, silanolates, phosphines,phosphine oxides, phosphates, amines, thiols, thioethers, disulfides,and sulfoxides. Examples of inorganic materials that may be used aspassivation layers include, but are not limited to, silicon containingmaterials, such as silicon nitride, silicon dioxide, and amorphoushydrogenated silicon carbide; Group II/VI materials such as ZnS, ZnO,and MgO; Group III/V materials such as BN and AlN; metal oxides, such asalumina, titanium dioxide, zirconium dioxide, strontium titanate, andyttrium aluminum garnet; and diamond. Core-shell nanoparticles having aSi core and a ZnS shell are one specific example of core-shellnanoparticles that may be made in accordance with the present methods.

Organic passivation of nanoparticles produced in the plasma may beaccomplished by exposing the nanoparticles to the passivating agentsunder appropriate conditions after the nanoparticles have exited theplasma reaction region. Since most organic passivating agents cannotsurvive the conditions of the plasma, it is generally desirable that theorganic passivation be performed in a subsequent step to avoiddecomposition of the passivating agents. This subsequent passivationstep may be carried out in a continuous flow process or a batch process,depending upon the configuration of the reaction system.

One method of passivating the nanoparticles with an organic passivationlayer is to introduce a gas-phase passivating agent into the aerosol gasstream. The reaction between the passivating agent and the surface ofthe nanoparticle may be mediated by thermal energy, for example byflowing the combined gas stream through a heated region, or it may becatalyzed by another reagent, for example a gas-phase Lewis acid orother catalytic agent. Another method for passivating the nanoparticleswith organic passivation layers (e.g., organic monolayers) can beperformed by capturing the nanoparticles in a liquid which is a suitablesolvent for the organic passivating agent. Examples of such solventsinclude, but are not limited to, alkanes, such as hexane; simplearylenes, such as benzene and toluene; ethers, such as diethylether andtetrahydrofuran; ketones, such as acetone; alcohols, such as ethanol,isopropanol and octanol; halogenated organics, such as chloroform,chlorobenzene and trichloroethylene; coordinating solvents, such astrioctylphosphine oxide, dimethylsulfoxide, or octylamine. Thenanoparticles may be captured in liquid by bubbling the aerosol throughthe liquid or by depositing the nanoparticles onto a substrate andimmersing the substrate in the liquid. In the later case, stirring orultrasonic agitation may be employed to disperse the nanoparticles intothe liquid. After transferring the nanoparticles into an appropriateliquid, reactions with organic passivating agents can be performed. Awide range of chemical reactions may be employed to derivatize thesurface of the semiconductor particles. Descriptions of such reactionsmay be found in J. M. Buriak, Chemical Reviews 102(5), pp. 1271-1308(2002), describing silicon and germanium surface derivatization andPeng, et. al. U.S. Patent Application Publication No. 2002/0066401 A1,describing derivatization of Group II/VI surfaces, the entiredisclosures of which are incorporated herein by reference.

Core-shell nanoparticles composed of a nanoparticle core and aninorganic passivating shell may be fabricated using a double plasmareactor process in which two plasma reactors are connected in series.The core nanoparticles are produced in the first of the two reactorsusing a RF plasma, which may be a constricted RF plasma, as describedabove. The nanoparticles so produced are then injected into the secondplasma reactor along with a gas containing passivation precursormolecules under conditions that promote the dissociation of the shellprecursor molecules. In some embodiments the shell precursor moleculesmay be pre-dissociated before they are introduced into the secondreactor. For example, a gas containing nitrogen and silane may be usedto form a silicon nitride passivation layer on a semiconductor (e.g.,Si) nanoparticle. A gas containing methane and silane may be used toform a silicon carbide passivation layer. Other passivation precursorgases may be used to produce alternate types of inorganic passivationlayers. Many of these gases are already employed in thin film depositionsystems, such as chemical vapor deposition (CVD) systems, and are knownto those skilled in the art.

During the double reactor production of the inorganically passivatednanoparticles, the pressure in the second plasma reactor will typicallybe lower than in the first plasma reactor. For example, when a totalplasma pressure of 10 Torr is used to form the core nanoparticles, thetotal gas pressure during the formation of the shell may be between 5-10Torr, and the partial pressure of the gas containing the passivationprecursor molecules may be between 10-200 mTorr depending on the desiredproperties of the passivation layer. Typical residence times are between0.1 and 1 seconds and typical RF powers are between 100 and 300 W.Secondary plasmas, such RF or DC glow plasmas, inductively coupled RFplasmas, microwave sustained plasmas, or any kind of sputtering plasmasuch as DC or RF magnetron plasmas may also be used to produce thepassivating shell layers. In some embodiments, the range of parametersfor these plasmas encompasses total gas pressures from 10 mTorr to 10Torr, precursor partial pressures from 1 mTorr to 1 Torr, and RF or DCpowers from 5 Watt to 3 kW.

In an alternative embodiments, core-shell nanoparticles composed of ananoparticle core and an inorganic passivating shell may be fabricatedusing a double reactor process in which a plasma reactor is connected inseries with a gas-phase reactor. Again, the core nanoparticles areproduced in the plasma reactor using a RF plasma, which may be aconstricted RF plasma, as described above. The nanoparticles so producedmay then be injected into the gas-phase reactor where an epitaxial shellmay be grown on the nanoparticles by gas condensation.

In one embodiment, a shell may be grown by introducing the nanoparticlesproduced in the plasma reactor and a precursor vapor into the gas-phasereactor under conditions that promote the decomposition of the precursorvapor molecules followed by the condensation of the decomposed specieson the nanoparticles. Typically the precursor vapor is delivered intothe reactor by an inert carrier gas, such as helium or argon. A reactivegas may be fed into the gas-phase reactor along with the precursor vaporand the carrier gas. For example, an organometallic precursor vapor maybe introduced along with oxygen in an inert carrier gas in order toprovide a metal oxide passivating shell. Alternatively, as in the caseof a metal shell, a metal vapor may be introduced into the gas-phasereactor from an evaporation source such that the resulting vapor-phasemetal atoms condense on the nanoparticles to provide a metal shell.Optionally, a reactive gas, such as oxygen or nitrogen, may beintroduced with the metal vapor to provide, for example, a metal oxideor metal nitride shell. Suitable vapor sources include, but are notlimited to, resistive heating vapor sources and electron beam vaporsources.

The growth of the shell in the gas phase will generally take place atlow pressures (e.g., pressures less than atmospheric pressure) andelevated temperatures. For example, in some illustrative embodiments,the pressure in the gas-phase reactor may be maintained at about 10⁻¹ to10⁻⁴ Torr and temperatures of at least about 300° C. Gas-phase synthesisof the nanoparticle shells may be particularly desirable whereagglomeration of nanoparticles in a double plasma reactor system is aproblem.

In yet other embodiments, core-shell particles may be formed in a singlestep plasma process using a mixture of two or more precursor gases. Twoprocesses may lead to the formation of core-shell particles. In thefirst process the partial pressure of a first precursor gas “A” isadjusted such that the precursor molecules in that gas are more amenableto particle nucleation than those of a second precursor gas “B”, and theformation of particles from A precursor molecules is thermodynamicallyfavored over the formation of compound particles (“AB” particles) from Aand B precursor molecules. These conditions lead to the initialnucleation of small particles (nuclei) from dissociated precursorspecies of the A molecules. On the surface of these particles a shellforms through condensation of precursor species from the B molecules orthrough condensation of the compound material AB. For example, a mixtureof silane and methane may be introduced into a plasma reactor. Silaneforms particles much more readily than methane. Hence these conditionslead to the nucleation of silicon particles which are then coated byeither a carbon or silicon carbide film, depending on the ratio of theprecursor gases used. Exemplary conditions for this process wouldinclude: total plasma pressure of about 1 to 5 Torr; Silane(SiH₄:He-5%:95%) flow rate of about 10 sccm, methane (CH₄:Ar-5%:95%)flow rate of about 40 sccm, and plasma power of 200 W. However, thepresent invention is not limited to processes conducted under theseconditions.

In the second process, use of two precursor gases “A” and “B” initiallyleads to the formation of particles of a compound material “AB”. If thecomposition of the AB particles is nonstoichiometric, a phasesegregation will occur under a sufficiently high temperature, leading toformation of an A core in an AB shell. In some embodiments thesufficiently high temperature is achieved under the conditions of theplasma in which precursor dissociation occurs. However, suitable thermalconditions for promoting phase segregation may also be achieved byheating the nanoparticles in a furnace, heating the nanoparticles with alaser, depositing the nanoparticles on a heated substrate, heating thenanoparticles with infrared radiation or heating the nanoparticles in aliquid or supercritical fluid environment. Thus, the phase segregationcould be performed either in flight in the gas phase, after depositiononto a solid substrate, or after collection in a liquid or fluid. Anexample of this process would be the formation of silicon core particleswith a silicon nitride shell. The particles initially formed in theplasma are a silicon-rich silicon nitride. At particle temperaturessignificantly above 300° C. in the plasma, a phase segregation willoccur which leads to the formation of a silicon core particle with asilicon nitride shell of reduced silicon content as compared to theinitial silicon nitride particle. Examplary conditions for this processwould include: Silane flow rate of about 10 sccm (or correspondinglyhigher if silane is diluted in a carrier gas), nitrogen flow rate of 100to 800 sccm, and plasma power of 300 W. However, the present inventionis not limited to processes conducted under these conditions.

In some instances, nanoparticles having both an inorganic shell and anorganic passivating layer may be produced. For example, a nanoparticlecore with an inorganic shell may be produced in accordance with themethods described herein (e.g., in a double plasma or a serialplasma/gas-phase reactor system) and the resulting nanoparticles maythen be exposed to liquid phase organic passivating agents to provide aprotective organic layer over the inorganic shell.

FIG. 11 shows a schematic diagram of a three phase system for makingpassivated nanoparticles. In the first phase, plasma precursormolecules, in this case SiH₄ molecules, are passed through a coreprecursor gas injection port 1101 into a first plasma reactor 1100 wherea radiofrequency plasma (e.g., a constricted RF plasma) is ignited toform crystalline nanoparticles 1102. These nanoparticles are then passedthrough connecting tubing 1103 into a second reactor 1104 along with apassivation precursor 1106, introduced upstream of the second reactorthrough a shell precursor gas injection port 1107, and an inorganicpassivating shell 1108 is grown on the nanoparticles. The second reactormay be either a plasma or a gas-phase reactor. In a third phase, theresulting core-shell nanoparticles are then passed into a solvent 1110containing organic passivating agents 1112 to provide an organicprotective shell on the inorganic core-shell nanoparticles. In oneembodiment of the system of FIG. 11, the first and second plasmareactors have a length of about 12 cm, the inside diameter of the firstplasma reactor is about 0.25 in. and the inside diameter of the secondreactor is about 1 inch.

An alternative embodiment of the first and second plasma reactors isshown in FIG. 20. In this system 2000, the connecting tubing 1103 iseliminated and the first plasma reactor 2002, including a pair of ringelectrodes 2003 and the second plasma reactor 2004, including a pair ofring electrodes 2005 are immediately adjacent, such that the plasmareaction zones overlap. As before, the core precursor gas may beintroduced into the first plasma reactor 2002 through a core precursorgas injection port 2006 and the shell precursor gas may be injectedthrough a shell precursor gas injection port 2008. In the embodimentdepicted in FIG. 20, the nanoparticles are collected on a collectionmesh 2010 located in or downstream of the second plasma reactor 2004.

Nanoparticles disposed on a substrate and coated with a passivating filmmay also be formed by a plasma process in which the nanoparticles areproduced in a first stage and the inorganic passivating film is formedover the nanoparticles in a second stage. In this case, thenanoparticles are typically prepared in a batch fashion, in which thenanoparticles are first formed in an RF plasma, as described above, thencollected in an appropriate manner, such as on substrate, and treatedwith a second plasma or a vapor containing passivation precursormolecules. Various suitable means for collecting the nanoparticles areknown. These include, but are not limited to, collection bythermophoresis on a cold surface, mechanical filtration andelectrostatic collection.

If the passivating film is formed from gas-phase (i.e., non-plasma)precursors, a precursor vapor containing precursor species and,optionally, a reactive gas may be introduced into the chamber underconditions which promote the condensation of the precursor species intoa passivating film over the nanoparticles. For example, CdSnanoparticles deposited onto a substrate may be coated with a protectivesulfur layer by passing a heated (e.g., to 300° C. or higher) gas of H₂Sover the substrate under conditions in which the H₂S decomposes andsulfur is deposited on the nanoparticles.

If a second plasma is used to provide the passivating film, a plasmasource gas containing inorganic precursor molecules is introduced intothe reactor. This second plasma is used to overcoat the nanoparticleswith an appropriate inorganic passivating material. There are at leasttwo approaches for forming a passivating film over nanoparticlesdisposed on a substrate using a double plasma technique. In a firstapproach a passivating film is deposited onto the nanoparticles byproducing a plasma discharge in a gas containing one or more passivationprecursor species to dissociate the precursor molecules. For example,silicon nanoparticles may be deposited onto a substrate and at leastpartially covered by a film of an inorganic passivating material such asamorphous silicon, amorphous silicon oxide, amorphous silicon carbide oramorphous silicon nitride. This may be accomplished by exposing thenanoparticles to a plasma ignited in a gas containing a mixture ofpassivation precursor molecules, such as silane or disilane and oxygen,methane, nitrogen or ammonia. The passivating film is deposited on thenanoparticles by dissociating these precursor molecules in the plasma.

In a second approach to forming a passivating film over nanoparticles ona substrate, a passivating film is formed by creating a plasma dischargein a gas containing one or more passivation precursor molecules tocreate reactive species, such as ions and radicals, in the plasma. Apassivating film is formed over the nanoparticles by reactions of thenanoparticles with the reactive species from the plasma. For example,silicon nanoparticles may be exposed to a nitrogen or oxygen plasma. Thenitrogen and oxygen radicals and ions will react with the silicon at thenanoparticle surface to form a passivating film of silicon nitride orsilicon oxide.

During the production of nanoparticles on a substrate that arepassivated by an inorganic film, the plasma may be sustained either byDC or RF power, and excitation mechanisms such as capacitively coupledRF, inductively coupled RF, or microwave excitation can be used.Precursor pressures are typically between 1 mTorr and 1 Torr, total gaspressures between 5 mTorr and 10 Torr, and RF or DC powers between 2Watts and several hundreds of Watts. The substrate temperature may bevaried over a wide range, but is typically between room temperature andabout 1000° C.

The nanoparticles may be deposited on a variety of substrates, includingboth organic and inorganic substrates. Deposition may take place priorto coating the nanoparticles with a passivating shell, as describedabove, or after the core-shell structures have been formed. Thenanoparticles exiting the plasma and/or gas-phase reactors may bedeposited onto a substrate by passing the stream of nanoparticles overthe substrate and allowing the nanoparticles to become affixed to thesubstrate by, for example, electrostatic attraction or inertialimpaction. In some such embodiments the substrate is a continuouslymoving substrate. In this embodiment the substrate may be provided as asheet of flexible material suspended between two rolls wherein the sheetis dispensed from the first roll and collected onto the second rollafter nanoparticle deposition. Because the plasma-produced nanoparticlesare characterized by charged surfaces, they are well adapted for thinfilm deposition. This is because the surface charges on thenanoparticles cause them to repel one another, resulting is a thin(e.g., single) layer of nanoparticles on the substrate rather than anuneven distribution of nanoparticle agglomerates on the surface. Inaddition, because the nanoparticles are so small, they may be suspendedin a relatively low density gas flow, making it easier, relative tolarger particles, to deliver a stream of the nanoparticles to asubstrate.

In embodiments where nanoparticles cores are first deposited onto asubstrate and subsequently coated with a passivating layer, thedeposition may take place in a two-step process where each step takesplace at a different location in the system. In one such embodiment, amoving substrate sheet (e.g., a sheet fed from a roll of substrate)crosses in front of the outlet of a plasma reactor where nanoparticlecores collect on the substrate. The substrate then passes through asecond reactor, which may be either another plasma reactor or agas-phase reactor as discussed above, where a passivating film is grownover the nanoparticles. In this embodiment, the substrate sheet may movein a continuous or a step-wise fashion. A roller system suitable for usein the present methods is described in U.S. Pat. No. 4,400,409, theentire disclosure of which is incorporated herein by reference.

Suitable substrates include both inorganic and organic substrates. Thesubstrates may be flexible or non-flexible. Flexible polymer substratesare particularly well-adapted for use in systems that employ acontinuously moving substrate. Suitable inorganic substrates includesemiconductor, glass and ceramic substrates including but not limitedto, silicon, quartz or pyrex and alumina substrates. Webs, such asaluminum or stainless steel webs, may also be used.

FIG. 12 shows a schematic diagram of a system that may be used for thecontinuous deposition of nanoparticles on a flexible substrate using aroll-to-roll substrate feed mechanism. This system is the same as thatof FIG. 11, described above, until the core-shell nanoparticles exit thesecond reactor. Upon exiting the second reactor however, thenanoparticles are shot out toward a continuously moving substrate 1200suspended between two rotating rollers 1202, 1204, rather than beingdirected into a solvent.

Any plasma reactor apparatus capable of producing a RF plasma (or in thecase of the constricted plasma-based methods, capable of producing aconstricted RF plasma) may be used to carry out the processes providedherein. However, the reactors are desirably flow-through reactorsbecause these allow for high mass throughput. In such reactors reactantsflow in through an inlet, nanoparticles are produced in a reactionchamber and the nanoparticles flow out, generally as an aerosol, throughan outlet. One illustrative example of a suitable plasma reactorapparatus is shown in FIG. 1. The apparatus 100 includes an inlet portin fluid communication with a plasma chamber 102. As shown in the figurethe plasma chamber 102 may take the form of a discharge tube such as aquartz discharge tube having a narrow inside diameter. An electrodeassembly 104 is situated in or around the plasma chamber 102. When an RFvoltage is applied across the electrodes in the electrode assembly 104in the presence of a precursor gas, a constricted plasma 105 may beformed in the plasma chamber 102. Nanoparticles 106 formed in theconstricted plasma 105 are passed out of the plasma chamber 102 in theform of an aerosol. In some embodiments, the apparatus includes anoutlet port. The outlet port may be a small aperture separating theplasma chamber 102 from a downstream chamber (not shown) having a muchlower internal pressure. Using this design, a jet containing thenanoparticles may be formed downstream of the aperture. Thenanoparticles in the jet may then be accelerated and deposited onto asubstrate (e.g., a TEM grid) housed within the downstream chamber.Optionally the apparatus 100 may be equipped with an evacuator/diluter108 in fluid communication with the plasma chamber 102. Theevacuator/diluter 108 may be used to bring the aerosol exiting theplasma chamber 102 up to atmospheric pressure. In the embodimentdepicted in FIG. 1, the apparatus also includes a neutralizer 110 influid communication with the evacuator/diluter 108. The aerosol exitingthe evacuator/diluter 108 may be passed into the neutralizer 110 inorder to impart a well defined charge distribution to the aerosol bycharging the nanoparticles with radiation from a suitable radioactivesource, such as polonium. The particles from the neutralizer 110 maythen be fed into a Differential Mobility Analyzer (DMA) 112 whichmeasures the size distribution of the nanoparticles. The nanoparticlesmay flow from the DMA 112 into a particle counter 116 which is in fluidcommunication with the DMA 112. A description of a DMA that may be usedto characterized the nanoparticles may be found in “A Nanometer AerosolSize Analyzer for Rapid Measurement of High Concentration SizeDistributions,” Journal of Nanoparticle Research, 2, 43-52 (2000), theentire disclosure of which is incorporated herein by reference.

FIG. 2 shows a more detailed view of the electrode assembly of theapparatus of FIG. 1. The electrode assembly 104 includes a pair of ringelectrodes 200 surrounding the plasma chamber 102. The ring electrodes200 are disposed in a generally parallel arrangement and separated by asmall gap 200. In one exemplary embodiment of the electrode assembly104, the ring electrodes 200 are copper electrodes separated byapproximately 0.5 inches. In the electrode assembly one of the two ringelectrodes serves as an RF powered electrode and the other ringelectrode provides a ground electrode. As discussed above when an RFvoltage is applied to the RF powered electrode in the presence of aprecusor gas, a RF plasma discharge may be generated between the tworing electrodes 200. In an alternative embodiment of the electrodeassembly depicted in FIG. 2, only one of the electrodes (e.g., the RFpowered electrode) is a ring electrode while the other electrode (e.g.,the ground electrode) is a plate electrode. However, other electrodeconfigurations may be employed.

An alternative plasma reactor assembly is shown in FIG. 13. Thisassembly utilizes a plurality of RF reactors operating in parallel toincrease nanoparticle production. As shown in the figure, this parallelplasma reactor includes multiple plasma chambers 1300 arranged withtheir plasma discharge axes in a substantially parallel alignment. Theplasma chambers are housed within a single vacuum vessel 1302. As shown,the plasma chambers may take the form of discharge tubes (e.g., quartzdischarge tubes) having narrow inside diameters while the vacuum vesselmay take the from of a larger insulating tube. The arrangement of theplasma chambers may vary within the vacuum vessel. For example, in oneexemplary embodiment the plasma chambers are arranged in a circulargeometry around the central longitudinal axis of the vacuum vessel. Theplasma chambers may have a common electrode assembly. In FIG. 13, theelectrode assembly includes two outside, grounded electrodes 1304 and acentral RF electrode 1306 through which each of the plasma chambers 1300passes. The RF power cable associated with a RF generator may beattached to the central electrode, for example, at the center of thedisc. A seal is formed between the outer electrodes and the vacuumvessel and the space between the RF electrode and each of the outerelectrodes is filled with an appropriate dielectric medium 1308, such aspolycarbonate, quartz or alumina. The dielectric medium prevents plasmaformation outside of the plasma chambers and may also be used to holdthe plasma chambers in place within the larger vacuum vessel. Duringoperation, a plasma precursor gas and any buffer gases flow into thevacuum vessel and into the discharge tubes. A RF voltage is then appliedacross the electrodes and a radiofrequency discharge is ignited andsustained in each of the tubes to produce single-crystal nanoparticles.These nanoparticles then flow out the opposite ends of the dischargetubes as a nanoparticle aerosol.

In one illustrative embodiment the vacuum vessel is a quartz tube havinga inside diameter of about 2 inches and each of the plasma chambers isquartz tube having an inside diameter of about 0.25 inches and anoutside diameter of about ⅜ inches. The dielectric is composed ofpolycarbonate and the two electrodes are copper discs having a diameterof approximately 2 inches which fit tightly into the vacuum vessel tube.The electrodes each define 8 holes with diameters of approximately ⅜inches through which the 8 plasma chamber tubes pass. It should beunderstood, however, that this embodiment is provided by way ofillustration only and the materials and dimensions for use in thereactor may vary from what is described here.

Yet another plasma reactor structure is shown in FIG. 14. This structureincludes a matrix of holes 1400 perforating a composite sheet made fromtwo electrodes 1402, 1404 separated by a dielectric material 1406. Thefirst of the two electrodes is an RF electrode 1402 and the second ofthe two electrodes 1404 is grounded. The dimensions of the perforationsand the composite sheet are selected such that when an RF voltage isapplied across the electrodes in the presence of a plasma precursor gas,a RF plasma is ignited in the holes. In this construction, the matrix ofholes provides a matrix of substantially parallel plasma reactionchambers. In a typical embodiment the electrodes are about 0.1 to 2 mmthick, the dielectric layer is about 5 to 50 mm thick and the holes havean inner diameter of about 2 to 20 mm. The center-to-center distancebetween the holes is typically about 4 to 40 mm. However, the presentinvention is not limited to structures having these dimensions. In theembodiment depicted in FIG. 14, the composite sheet is housed within avacuum chamber 1408 and separates an input chamber 1410 from which aplasma precursor gas and any buffer gases flow into the holes and anoutput chamber 1412 into which an aerosol of nanoparticles is injectedfrom each of the plasma chambers (i.e., holes). Examples of suitablematerials for the electrodes are metals, such as copper and, inparticular, thin metal foils. Examples of suitable dielectric materialsinclude, but are not limited to, ceramics. The operating parameters(e.g., absolute pressure, partial pressures of the reactants and flowrates) for this apparatus may be the same as those discussed elsewherein this disclosure. However, methods for producing nanoparticles usingthis apparatus are not limited to those having operating parameters thatfall within these ranges.

EXAMPLES

Exemplary embodiments of the processes for producing crystallinesemiconductor nanoparticles are provided in the following examples. Theexamples are presented to illustrate the present processes and to assistone of ordinary skill in using the same. The examples are not intendedin any way to otherwise limit the scope of the invention.

Example 1 Production of Single-Crystal Silicon Nanoparticles Having aParticle Size of Approximately Ten Nanometers or Less

Using the experimental apparatus depicted in FIGS. 1 and 2,single-crystal silicon nanoparticles were generated. The electrodes inthe electrode assembly were copper ring electrodes composed of a copperwire wound about the outside of a quartz tube plasma chamber having aninside diameter of ¼ inch. A mixture of silane in helium(SiH₄:He=5%:95%) was used as a precursor gas and argon was used as abuffer gas. The plasma generation conditions used in this exampleincluded a chamber pressure in the presence of the plasma of 13 Torr, anargon flow rate of 200 sccm, a SiH₄:He flow rate of 0.31 sccm, and an RFpower of 120 Watts. When the constricted RF plasma was creatednanoparticles were formed through the dissociation of silane intosilicon and hydrogen, followed by nucleation of silicon particles in theconstricted plasma. The plasma was run for approximately 60 seconds,during which a narrow striated current channel (i.e., filament) havinghigh density plasma nodes could be observed inside the quartz tube. Thecurrent channel rotated in an annular region close to the quartz tubewalls at a frequency of about 800 Hz. It is estimated that thenanoparticle resonance time in the annular plasma regions was on theorder of about 4 milliseconds, during which time the nanoparticles wereexposed to the current channel numerous times. In order to collect thenanoparticles for analysis, a TEM grid was placed in front of theevacuator/dilutor.

A TEM analysis of the nanoparticles was performed on a high resolutionJEOL 1201 transmission electron microscope operating at an acceleratingvoltage of 120 kV. The TEM image of the silicon nanoparticles is shownin FIG. 3. The nanoparticles are crystalline nanoparticles, as evidencedby the diffraction pattern shown in FIG. 4. The primary particlediameter for the silicon nanoparticles is approximately 10 nm. The sizedistribution for the nanoparticles measured with a DMA is shown in FIG.5. The distribution is relatively monodisperse, centered atapproximately 13.6 nm mean particle diameter with a standard deviationof about 5.8 nm.

The image of FIG. 3 indicates that there may be some agglomeration ofthe silicon nanoparticles deposited on the TEM grid. This agglomerationmay occur as the nanoparticles are deposited on the TEM grid.

Example 2 Production of Single-Crystal Silicon Nanoparticles Having aParticle Size of Approximately Five Nanometers or Less

The experiment of Example 1 was repeated using the following modifiedplasma generation conditions: the chamber pressure in the presence ofthe plasma was 11 Torr, the flow rate of SiH₄:He was increased to 0.34sccm and the RF power applied to the RF powered electrode was decreasedto 110 Watts. The plasma was run for approximately 60 seconds, duringwhich a narrow striated current channel (i.e., filament) having highdensity plasma nodes could be observed inside the quartz tube. Thecurrent channel rotated in an annular region close to the quartz tubewalls at a frequency of about 800 Hz. It is estimated that thenanoparticle residence time in the annular plasma regions was on theorder of 4 milliseconds, during which time the nanoparticles wereexposed to the current channel numerous times. In order to collect thenanoparticles for analysis, a TEM grid was placed in front of theevacuator/dilutor.

FIG. 6 shows the particle size distribution for the crystalline siliconnanoparticles obtained in this example. The distribution is relativelymonodisperse, centered at approximately 6.2 nm mean particle diameterwith a geometric standard deviation of about 1.8 nm. The sizedistribution shown in FIG. 6 reflects nanoparticles having a primaryparticle size of about 3-5 nm which have undergone some moderateagglomeration after leaving the plasma. FIG. 7 shows the TEM image of asingle-crystal silicon nanoparticle produced in accordance with thisexample. The TEM images were obtained with a high resolution JEOL 2010Ftransmission electron microscope operating at an accelerating voltage of200 kV. The silicon nanoparticle shown in FIG. 7 has a particle diameterof less than about 5 nm.

Example 3 Production of Single-Crystal Silicon Nanoparticles Having aParticle Size of Approximately 30 to 50 Nanometers

Single-crystal silicon nanoparticles were formed using the apparatusshown in FIGS. 1 and 2 with the modification that a copper plateelectrode was used as the ground electrode rather than a copper ringelectrode and a quartz tube having a 1.85 inch inside diameter, ratherthan a ¼ inch inside diameter was used. The two electrodes wereseparated by a distance of approximately 10-15 cm. A mixture of silanein helium (SiH₄:He=5%:95%) was used as a precursor gas and argon wasused as a buffer gas. The plasma generation conditions used during theformation of the silicon nanoparticles included a chamber pressure ofapproximately 1.5 Torr, an argon flow rate of 3 sccm, a SiH₄:He flowrate of 2.5 sccm and an RF power input of 200 Watts. The plasma was runfor approximately 90 seconds, during which a narrow striated currentchannel (i.e., filament) having about 15 high density plasma nodes couldbe observed inside the quartz tube. The current channel rotated in anannular region close to the quartz tube walls at a frequency of about150 Hz. It is estimated that the nanoparticle resonance time in theannular plasma regions was on the order of four seconds, during whichtime the nanoparticles were exposed to the current channel numeroustimes. An aerosol containing single-crystal silicon nanoparticles formedin the quartz tube was extracted by passing the aerosol through a 1 mmextraction orifice. The pressure in a low pressure chamber downstream ofthe extraction orifice was 10⁻³ Torr. This pressure difference led tothe formation of a supersonic gas jet downstream of the orifice.Particles exiting the quartz tube were accelerated in this jet tovelocities of up to 250 m/s and are deposited onto a substrate byinertial impaction.

TEM analysis of the nanoparticles was performed on a Philips CM30transmission electron microscope operating at an accelerating voltage of300 kV. FIG. 8 shows a TEM image of some of the silicon nanoparticles.Most of the deposited nanoparticles were cubes with the faces along the[100] planes. The difference in the apparent contrast of the particlesis due to small differences in the alignment of the particles withrespect to the electron beam. Aligned particles show strong Braggdiffraction and appear dark. The silicon nanoparticles weresingle-crystal diamond-cubic silicon as verified by selected-areaelectron diffraction (not shown). FIG. 9 shows a high resolution TEMimage of an individual silicon nanoparticle (circled). No evidence ofdislocations or other planar defects in the crystalline nanoparticle wasfound. A 1-2 nm thin amorphous layer surrounds the particle. This layeris most likely an oxide surface layer formed on exposure of thenanoparticles to air.

FIG. 10 shows the particle size distribution as determined from imageanalysis of the TEM images. Using the software NIH Image J, the particlediameter is obtained from the TEM images by assuming the particle areato be the projection of a spherical particle. The particle sizedistribution shown in FIG. 10 is based on the analysis of more than 700particles. The distribution is rather monodisperse, centered at 35 nmmean particle diameter with a geometric standard deviation of 1.3. Thecrystallinity of the particles implies a high gas temperature.

Example 4 Production of Photoluminescent Single-Crystal SiliconNanoparticles

In this example photoluminescence was observed for single-crystalsilicon nanoparticles obtained with the apparatus of FIGS. 1 and 2. Thenanoparticles were deposited on a quartz tube serving as substratethrough thermophoretic deposition, which relies on the so calledthermophoretic force which in the presence of a temperature gradient inthe gas pushes particles from hot to cold regions. Liquid nitrogen wasflowed through the quartz tube in order to cool it to well below roomtemperature. The gas emanating from the plasma reactor was attemperatures higher than room temperature leading to deposition of theparticles on the tube. After deposition the nanoparticles were removedfrom the chamber and exposed to the atmosphere. Upon exposure to theatmosphere, the particles exhibited photoluminescence.

To collect the photoluminescence spectrum, the nanoparticles were placedin front of the entrance slit to a monochromator (ISA Instruments modelHR-320). The nanoparticles were then illuminated using UV light from alamp emitting at 365 nm (UVP model: UVL-56). The UV light from the lamppassed through a low pass filter (Nikon model UR-2). The siliconnanoparticles emitted light via photoluminescence (PL) under UVillumination. The PL light passed into the entrance slit of themonochromator. The monochromator only allowed light of a givenwavelength to pass through it. At the exit of the monochromator, thelight was detected by a photomultiplier (Hamamatsu model: C659-71). Theelectrical signal generated by the photomultiplier was recorded by aphoton counter (Hamamatsu model: C5410) which converted the electricalsignal from the photomultiplier into photon counts. The photon counterthen sent the photon counts to a computer via RS232 communication forstorage. The monochromator was scanned through portions of the visiblespectrum using a computer control unit (Spectra-Link, ISA Instruments).

The nanoparticles produced in accordance with the methods described inthe patent were found to exhibit photoluminescence. Thephotoluminescence usually became more intense the longer the particleswere exposed to the atmosphere. Particles produced at the conditionsoutlined in Example 1 and Example 2 above exhibit photoluminescence.Particles, which were produced at conditions under which a “rotatingconstriction” was present, usually exhibited much more intensephotoluminescence than samples produced when the constriction waspresent but not rotating. This may be because a smaller fraction of theflow passes through the constriction, which may cause a smaller portionof the particles to be crystalline. It has been observed that particlesproduced at different experimental conditions exhibit photoluminescenceat different wavelengths. Visible photoluminescence has been observed ina range from ˜450 nm to more than 775 nm.

Silicon nanoparticles made in accordance with this Example weredeposited on quartz tubes. The nanoparticles were observed tophotoluminesce when excited by a UV lamp (at 360 nm) sitting below thetubes.

Example 5 Production of Single-Crystal Silicon Nanoparticles in aNon-Constricted RF Plasma

Using the same experimental apparatus described in Example 1,photoluminescent single-crystal silicon nanoparticles were generated ina non-constricted radiofrequency plasma. The use of a non-constricted RFplasma allows for the production of silicon nanoparticles atconsiderably lower pressures. A mixture of silane in helium(SiH₄:He=5%:95%) was used as a precursor gas and argon was used as abuffer gas. The plasma generation conditions used in this exampleincluded a chamber pressure in the presence of the plasma of about 1.5Torr, an argon flow rate of about 30-50 sccm, and a SiH₄:He flow rate of10 sccm. When the RF plasma was created nanoparticles were formedthrough the dissociation of silane into silicon and hydrogen, followedby nucleation of silicon particles. In order to collect thenanoparticles for analysis, a TEM grid was placed in front of theevacuator/dilutor.

A TEM analysis of the nanoparticles was performed on a high resolutionJEOL 1201 transmission electron microscope operating at an acceleratingvoltage of 120 kV. Two TEM images of the silicon nanoparticles are shownin FIGS. 15 and 16. The figures are dark field images wherein siliconsingle crystals are illuminated as bright points against the darkbackground of the micrograph.

The image of FIG. 16 indicates that there may be some agglomeration ofthe silicon nanoparticles deposited on the TEM grid. This agglomerationmay occur as the nanoparticles are deposited on the TEM grid.

Silicon nanoparticles made in accordance with this example weredispersed in methanol and showed to be photoluminescent. Thephotoluminescence spectra for these nanoparticles are shown in FIG. 17.

Example 6 Production of Core-Shell Single-Crystal Silicon Nanoparticles

This example describes the synthesis of core-shell silicon nanoparticlesusing a two-stage plasma reactor. Crystalline Si nanoparticles wereproduced in the first stage using 40 W of RF power, 145 sccm of Ar, and5 sccm of SiH₄ (10% in Ar). Conditions for the second plasma stage were:5 sccm of SiH₄ (10% in Ar), 5 sccm NH₃, using 40 W of RF power. Thepressure in both stages was about 6 Torr. An approximately 3 nm thickshell layer of SiN_(x) was deposited on the nanoparticles. A micrographof the nanoparticles is shown in FIG. 18. Luminescence was not observedfrom the particles, since these particles were deliberately formed toobig in order to make TEM analysis easier.

Relatively strong luminescence could be observed, however, by using thefollowing method of nanoparticle passivation: particles produced in thefirst stage were collected on a mesh that was placed at the end of thesecond stage. During particle deposition for 20 min., (1st stageconditions=20 W of RF power, 50 sccm of Ar, and 10 sccm of SiH₄ (5% inHe)) the second stage was turned off. The pressure was about 1.4 Torr.After deposition, the second stage was turned on and a pure NH₃ plasmawas run at 100 W, 100 sccm NH₃ for 30 min. Again, the pressure was about1.4 Torr. Under these conditions, the 2nd stage NH₃ plasma was in directcontact with the particles collected on the collection mesh. A SiN_(x)(silicon nitride)layer was formed by direct nitridation of the siliconparticles. The photoluminescence spectrum for the crystalline Sinanoparticles is shown in FIG. 19.

As a screening test for both cases, nanoparticles were exposed to KOHand were stable in the base environment. In contrast, Si and SiO₂nanoparticles are etched very rapidly by the base solution.

For the purposes of this disclosure and unless otherwise specified, “a”or “an” means “one or more”. All patents, applications, references andpublications cited herein are incorporated by reference in theirentirety to the same extent as if they were individually incorporated byreference.

The invention has been described with reference to various specific andillustrative embodiments. However, it should be understood that manyvariations and modifications may be made while remaining within thespirit and scope of the invention.

1. A method for producing semiconductor nanoparticles, comprising:providing a continuous flow of a gas containing asemiconductor-containing precursor monomer through a plasma reactorhaving an inlet and an outlet; applying a high frequency voltage betweentwo electrodes arranged proximate the plasma chamber and therebycreating a capacitively coupled plasma in the plasma reactor;dissociating the semiconductor-containing precursor in the plasma toprovide precursor species that nucleate and grow into semiconductornanoparticles wherein; the precursor is primarily converted intononagglomerated semiconductor nanoparticles in the high-frequencycapacitively coupled plasma, and the nanoparticles are carried out ofthe reactor through the outlet in a continuous fashion.
 2. The method ofclaim 1, further comprising depositing the semiconductor nanoparticlesonto a substrate.
 3. The method of claim 2, wherein the nanoparticlesare deposited onto a substrate that moves in a continuous or step-wisefashion to produce a film of nanoparticles on the substrate.
 4. Themethod of claim 1, wherein the semiconductor nanoparticles comprisesingle-crystal semiconductor nanoparticles.
 5. The method of claim 4,wherein the single-crystal semiconductor nanoparticles have an averagediameter of no more than about 50nm.
 6. The method of claim 1, whereinthe semiconductor nanoparticles comprise Group IV semiconductornanoparticles.
 7. The method of claim 6, wherein the Group IVsemiconductor nanoparticles comprise silicon nanoparticles.
 8. Themethod of claim 1, wherein the semiconductor nanoparticles compriseGroup II-IV semiconductor nanoparticles.
 9. The method of claim 8,wherein the Group II-VI semiconductor nanoparticles comprise CdSenanoparticles.
 10. The method of claim 1, wherein the semiconductornanoparticles comprise Group III-V semiconductor nanoparticles.
 11. Themethod of claim 1, wherein the semiconductor nanoparticles arephotoluminescent nanoparticles.
 12. The method of claim 1, wherein thetotal pressure in the plasma reactor during the production of thesemiconductor nanoparticles is less than about 5 Torr.
 13. The method ofclaim 1, wherein the total pressure in the plasma reactor during theproduction of the semiconductor nanoparticles is less than about 3 Torr.14. The method of claim 1, wherein the semiconductor nanoparticles havean average diameter of no more than about 10nm.
 15. The method of claim1, wherein the semiconductor nanoparticles have an average diameter ofno more than about 5nm.
 16. The method of claim 1, wherein at least oneelectrode comprises a ring electrode.
 17. The method of claim 1, whereinat least one electrode comprises a plate electrode.
 18. The method ofclaim 1, wherein dissociating the semiconductor precursor includesgenerating monodispersed nanoparticles.
 19. The method of claim 1,wherein the semiconductor nanoparticles comprise a mixture of elementsfrom different groups of the periodic table.
 20. The method of claim 1,wherein at least one electrode surrounds a dielectric discharge tube.21. The method of claim 1, wherein the plasma reactor includes aplurality of inlets.